libgcrypt: detect libgpg-error with pkg-config

[Niibe Yutaka]

Vincent Torri wrote:
> and about using pkg-config instead to detect libgpg-error ?

The configure script of libgcrypt is written for use of gpgrt-config.
It's same for other libraries of GnuPG, and GnuPG itself.

gpgrt-config is developed to replace many *-config for GnuPG
(gpg-error-config, libgcrypt-config, libassuan-config, libksba-config,
and npth-config, etc.).  gpgrt-config is a minimum subset of pkg-config.
"Minimum" means that, it should have enough but no other features to
support GnuPG build.

The intention is to allow building GnuPG without pkg-config.  GnuPG
build could be done in an early stage of OS porting, so, we pursue
less dependency other than GNU toolchain.

True, technically, it is possible to modify those configure scripts
to use pkg-config instead of gpgrt-config.  But our intention is
use of gpgrt-config here.

We (GnuPG team) don't have any plan to modify those configure scripts
with pkg-config.
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